Add support for remaining raster file formats

[cstephen]

I added raster processing support for just PNG and GIF because those were the two types of raster files I was seeing, but surely we'll need to include support for other types of rasters too (TIF, maybe even JPG?). But we should do some research to find out what formats are legitimate before throwing them all in there.

[cstephen]

The KML structure chart on this page will help us determine what other types of raster layers need plucked out of each KML file:

https://developers.google.com/kml/documentation/kmlreference

This will end up being relevant for issue #9, too, but less urgently since unKML currently allows all KML files to pass through to the ogr2ogr conversions step.